I’ve started using OmniOutliner with a single document, then split it into few separate files as it grown. I’ve found switching between a few open documents cumbersome. OmniFocus follows the old Mac convention, requiring a new application window for each open file. It would be a great addition to my workflow to have document switching available inside one window.
Here’s my concept of this feature, with a second sidebar, being a top level context above the sections: